The remote host is missing an update to courier announced via advisory DSA 533-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20533-1

A cross-site scripting vulnerability was discovered in sqwebmail, a web mail application provided by the courier mail suite, whereby an attacker could cause web script to be executed within the security context of the sqwebmail application by injecting it via an email message. For the current stable distribution (woody), this problem has been fixed in version 0.37.3-2.5. For the unstable distribution (sid), this problem has been fixed in version 0.45.4-4. We recommend that you update your courier package.